Microwave radiation can be applied to a material in a number of ways, using single mode, multimode applicators, traveling wave applicators, slow wave applicators, fringing field applicators and through free space. Each of the aforementioned methods of coupling microwave energy into a material has its advantages and disadvantages which usually depend on the dielectric properties, size and shape, of the materials to be heated and the type of processing (batch, continuous, . . . etc.) to be performed.
Efficient microwave energy transfer is a function of many variables as processing occurs. A number of these variables are material related, e.g., the material type and density and material temperature as well as the time history of both the material temperature and the applied electric field. As the material is heated, the dielectric constant may exhibit hysteresis in temperature and electric field strength. Depending on the nature of the change of the dielectric constant, this may result in the application of a non-uniform electric field or thermal runaway, e.g., hot and cold spots within the material or a mismatch between the resonant frequency of the loaded applicator and the microwave energy source. As the dielectric properties of the load change, the properties of the loaded applicator also change, for example, the resonant frequency of the applicator may change.
Other factors that influence coupling are related to the applicator, material geometry and size and the frequency or wavelength of the electromagnetic energy. Electromagnetic coupling depends on applicator size and geometry, material size and shape, the position of the material within the applicator, and even the relative sizes and shapes of the material and the applicator. In addition, both the applicator and material dimensions may change during heating which further complicates the efficient transfer of energy to the material.
Accordingly, a problem arises when attempting to maintain efficient coupling of the microwave energy into the applicator. This is especially difficult with a single mode, high Q factor applicator. This type of applicator can be tuned to specific electric field patterns (resonance modes) by varying the volume of the applicator. This, however, is laborious and provides difficult process control.
One such approach is found in U.S. Pat. Nos. 4,507,588, 4,585,668, 4,630,566, 4,727,293, and 4,792,772 (Asmussen), all of which disclose methods and apparatuses in which a single mode resonant microwave applicator can be critically coupled by varying two separate, almost orthogonal variables, specifically the cavity length (by moving a short circuit) and the antenna position.
The Asmussen devices include a variable penetration antenna structure which acts to launch radiation into the applicator. The main advantage of the Asmussen device is that it enables complete critical coupling over a wide range of impedance's (generated by the load in the applicator) and without the use of any external coupling structure. Critical coupling can thus be achieved by moving the short and the antenna appropriately.
By moving the flat part of the cavity wall (in a cylinder) in the z-direction (e.g., along the centerline of the cylinder), a wide range of electromagnetic modes can be established and maintained, even as the load varies (due to processing, e.g., temperature changing, material curing, etc.) As a result, if the load changes during processing (e.g., the dielectric properties change, due to increased temperature, curing, phase change in the material and so forth), the resonant frequency in the cavity changes from an initial, fixed processing frequency, usually 2450 MHz or 915 MHz (which are the ISM bands allowed by the Federal Communication Commission (FCC)). It should be understood that the output frequency of a magnetron, by far the most common microwave generating device available, is relatively narrow and is a fixed frequency (because the device itself is a resonant device) and cannot be conveniently varied over a wide range of frequencies dynamically.
U.S. Pat. No. 5,471,037 (Goethal) discloses a single mode cylindrical applicator that operates in the TM.sub.02n resonant mode. The microwave applicator is designed to process monomers in order to produce prepolymers. The size of the microwave applicator is selected according to the particular monomers being processed (e.g., fixed dimension applicator). Therefore, there is no mechanism for altering the diameter of the applicator to account for substantially different loads or substantially different dielectric properties.
U.S. Pat. No. 3,461,261 (Lewis) relates to a TM.sub.02n applicator that processes threads and yarns with the workpieces passing along the central axis of the applicator. The dimensions of the microwave applicator are selected according to the materials being processed (e.g., fixed dimension applicator).
The electric field pattern sustained by the TM.sub.0y0 series of modes, where y=1, 2 or greater, is oriented along the z-axis of the applicator and is of constant intensity along the entire length of the applicator for an empty cavity. This is an ideal mode for the processing of wide web-like materials. Referring to FIG. 1 (a mode chart), it can be seen that the TM.sub.010 mode is independent of the cavity length. Therefore, a low loss, infinitely long applicator is capable of sustaining the same electric field intensity throughout the length.
In all of the aforementioned prior art, all of the available methods rely upon a mechanical change of dimension of the applicator to match the resonant frequency of the applicator with the output frequency of the microwave source. This can be accomplished through the use of stepper motors and sometimes complex, computer driven feedback loops and are slow because of the time required for a motor to complete motion before another iterative measurement of the degree of tune is taken and the tune further improved.
Accordingly, an object of the present invention is to provide a means to maintain maximum coupling efficiency of microwave energy to a microwave applicator through the use of controlled feedback of critical material and process parameters.
It is a further object of the invention to provide a microwave applicator whereby field interactions with the material being processed are controlled through matching of the output frequency of the microwave source to the loaded applicator.
Another object of the invention is to provide a means for only allowing the desired mode and therefore electric field pattern within a loaded microwave applicator automatically through the use of a controlled feedback of critical material and process parameters.